Discharging a Capacitor

[pprendeville]

Want to test a capacitor with a Digital Multimeter butwant to take the right precautions beforehand. I've taken this PCB (is that the right term) that you can see in the pic from an old photocopier and want to mess around with components to familiarise myself with them. I'm not sure do i need to discharge the capacitor or not. The photocopier hadn't been used for about a year. Read the following in my course notes:

"Discharge the capacitor and disconnect one side of the capacitor to isolate it from the circuit. Some capacitors may have a discharge resistor that needs to be disconnected from the capacitor for the test."

Will capacitors remain charged for along long time or am I safe to go ahead and test it's resistance?

 

[adam.h]

Those caps are tiny, they will discharge pretty quickly. What 'tests' do you want to do on them? To measure the capicatance using a dmm you will need to remove it from the circuit.

 

[ian.settle1]

It looks like a electrolytic capacitor if it is make sure you connect it the right way round or else it will explode as they are polarity conciense

 

[adam.h]

thats the fun part lol.

I remember in electronics class we used to swap them all round on each others breadboards. Or charge a few up together then throw them at each other. Only a small shock but was still funny.

 

[pprendeville]

Those caps are tiny, they will discharge pretty quickly. What 'tests' do you want to do on them? To measure the capicatance using a dmm you will need to remove it from the circuit.

Want to do check to see if they're still working. According to my course notes "
A good capacitor has very high resistance of the order of megohms; a short-circuited capacitor has zero ohms resistance, and a leaky capacitor has a resistance lower than normal."

Here's the deal.

1. Connected +ve DMM probe to +ve wire on capacitor and -ve DMM probe to -ve capacitor wire and got a reading of one. DMM was set to 200K.
2. Switched around leads on DMM, +ve to -ve and vica versa, and I got a -ve reading of thirty which quickly went to zero and then started to increase to 100.

Took the probes away in case it exploded. What was happening here exactly?

 

[pprendeville]

It looks like a electrolytic capacitor if it is make sure you connect it the right way round or else it will explode as they are polarity conciense

How can you determine the type of capacitor if it just had a voltage and capacitance on it? Just in case it comes up in the future and I don't want to look foolish.

 

[adam.h]

A capicator is like an open circuit, but even free air has a limited resistance. plus with emf etc you will get leakage, so in theory yes, you should have a high resistance if there is no breakdown in the cap.

When you measure the resistance of a cap the DMM will uses ohms law to calculate, so will give out a voltage at a given current. But as the caps store charge the resistance will change which is what you are seeing.

You want a scope so you can see the charge and discharge

These links might help.

Charging a Capacitor

Capacitor - Wikipedia, the free encyclopedia

How can you determine the type of capacitor if it just had a voltage and capacitance on it? Just in case it comes up in the future and I don't want to look foolish.

the ones you see have a stripe down them, they state which side is negative, or sometimes positive.

like this...

http://img-europe.electrocomponents.com/largeimages/R519377-01.jpg


and this...

http://img-europe.electrocomponents.com/largeimages/R4963551-06.jpg

then there are non electrolytic that can connect up either way

like these...

http://img-europe.electrocomponents.com/largeimages/R4648375-01.jpg

and these

http://img-europe.electrocomponents.com/largeimages/R204035-05.jpg

 

[pprendeville]

Loadsa great info there. There are some of those non-electrolytic yellow capacitors on the circuit boards alright. You'd never know when that info will be useful to me. Going to read up on the theory in those sites and input values in the hyperphysics site also. thanks very much.

 

[ian.settle1]

thats the fun part lol.

I remember in electronics class we used to swap them all round on each others breadboards. Or charge a few up together then throw them at each other. Only a small shock but was still funny.

Remember connecting a electronic circuit for an experiment which had electrolytic capacotors, the lecturer had to check it before connecting power to the circuit.

Told us it was OK so switched on power next thing the was a loud bang and bits paper floating about in the air lecturer said should have looked at that a bit closer.

 

[SirKit Breaker]

I discharged one with the back of my hand recently it hurt like (NO SWEARING), i didnt mean to do it though.
When i was on the planes the engine igniters could hold a whacking punch, enough to throw you on the floor and make your eyes roll, we had some great times throwing them to the airframe fitters or putting them in their pockets

 

[kung]

many moons ago in avionics at rockwell collins when i was a tech we used to charge the big caps and leave them on the bench and get the trainees to go fetch lol